Several processes are known for reacting olefins with an organic hydroperoxide to produce oxirane compounds. For example, the process of Smith, U.S. Pat. No. 2,754,325, employs water soluble heteropoly acids containing the transition metals chromium, molybdenum or tungsten as homogeneous catalysts for the epoxidation of olefins in the presence of peroxides, which may be organic hydroperoxides, in an aqueous system. The use of solutions of transition metal compounds as homogeneous epoxidation catalysts is described in U.S. Pat. No. 3,350,422 and U.S. Pat. No. 3,351,635 of Kollar, which also provide a detailed description of the state of the art as of about 1966. Still another catalyst system useful for epoxidizing olefins is that described by deRoch et al. in U.S. Pat. No. 3,489,775, wherein the catalyst consists of salts of a nitrogenous organic base and an acid of molybdenum, tungsten or vanadium.
It is also known to employ heterogeneous catalyst to catalyze the epoxidation of an olefinically unsaturated material by reaction with an organic hydroperoxide.
All of these processes, while offering a distinct advantage over the older chlorohydrin process, suffer from the disadvantage that there is less than complete conversion of the organic hydroperoxide reactant. Consequently, the effluents from these epoxidation reactions (epoxidates) contain organic hydroperoxides. Hydroperoxides are known to decompose, thermally or in the presence of transition metal ions, into free radicals which, in turn, promote undesirable reactions in the subsequent processing of the epoxidates. For example, it has been found that thermal processing, as in distillation, of a propylene epoxidate which contains ethylbenzene and ethylbenzene hydroperoxide results in the formation of C.sub.5 - C.sub.7 hydrocarbons and similarly, a propylene epoxidate which contains propylene and t-butyl hydroperoxide results in the formation of undesired C.sub.7 and other light hydrocarbons. Such hydrocarbons are highly undesirable contaminants since they are difficult to separate by conventional fractionation techniques.
More specifically, in recovering propylene oxide by distillation from an epoxidate derived from the epoxidation of propylene with ethylbenzene hydroperoxide in the presence of a hetergeneous catalyst, sizeable concentrations of C.sub.5 - C.sub.7 hydrocarbons, e.g., 2,000 ppm, remain in the propylene oxide product fraction. Such a product is generally considered unsuitable for commercial purposes. Since an analysis of the epoxidation reaction effluent typically shows less than 10 ppm of such contaminants, and since their concentration in the propylene oxide product cannot be explained on the basis of mere concentration effect, it is apparent that such hydrocarbon contaminants are formed during processing of the epoxidate in the downstream product recovery system.
Therefore, there is a need for a method of converting the unreacted organic hydroperoxides present in a propylene epoxidation reaction effluent into stable compounds in order to prevent, or at least minimize, the decomposition of the peroxides into free radicals with the consequent hydrocarbon formation. Such conversion is preferably conducted before such epoxidate is subjected to the successive fractionations of the product recovery system.
One solution to this problem is presented in U.S. Pat. No. 3,464,897 of Jubin wherein a process is disclosed for distilling an epoxidate in the presence of a suitable azeotroping agent, e.g., an open chain or cyclic paraffin containing from 8 to 12 carbon atoms. Such an azeotropic distillation effectively depresses the volatilities of the C.sub.5 - C.sub.7 hydrocarbons, thus permitting the recovery of substantially pure propylene oxide. However, such an approach is expensive and presents an additional problem in recovering the azeotroping agent from the extracted hydrocarbons.
Numerous acidic catalysts or soluble metal catalysts have been disclosed for the decomposition of organic hydroperoxides or their conversion into other compounds. Typically the catalysts or reactants employed are acids or ionized metal compounds in solution. The mechanism for acid and ionized metal-induced decompositions of hydroperoxides is discussed in Tobolsky et al, "Organic Peroxides," New York: Interscience, 1954, pp. 57-122 and Davies, "Organic Peroxides," London: Butterworths, 1961, pp. 174-192.
It has also been taught that metals and compounds of metals of Groups IV-A, V-A, or VI-A of the Periodic Chart, with the exception of chromium, catalyze the conversion of alkenyl hydroperoxides to epoxy alcohols (Allison et al, U.S. Pat. No. 3,505,360, issued Apr. 7, 1970).
The use of acidic or soluble metal catalysts of the prior art to effect decomposition of residual organic hydroperoxide present in the effluent from a propylene epoxidation reactor would result in undesirable reactions which adversely affect the yields and the economics of the process. Usually, the organic hydroperoxide will be present in only minor concentrations, the major constituents of the mixture comprising propylene, propylene oxide, alcohols, carbonyl compounds, the hydrocarbon precursor of the hydroperoxide, and the like. The use of acidic or soluble metal compounds as catalysts effect the transformation of the propylene oxide into glycols, and aid in the formation of various epoxide adducts, and may induce the polymerization of propylene to heavy ends of little commercial value.
Thus, a real need exists for a method by which organic hydroperoxides present in a mixture of propylene, propylene oxide, alcohols, carbonyl compounds, hydrocarbons, and the like found in a propylene epoxidation effluent may be effectively decomposed without inducing reactions or decompositions of the admixed organic compounds.